African trypanosomes are highly antigenically variable flagellated protozoa that cause fatal disease of people and domestic animals. About 300,000 persons have trypanosomiasis at present and the disease threatens an additional 60 million individuals in sub-Saharan Africa as well as severely limiting agricultural development in an area the size of USA. African trypanosomes replicate by binary fission in the blood and interstitial fluids and are controlled by host antibody responses against their variable coat antigens. Infected hosts lose the ability to mount an effective antibody response as the disease progresses resulting in poor control of newly arising trypanosome antigenic types, sustained pathology and eventually death. Our long term goal is to understand how T. brucei affects the B cell (antibody forming cell) system and to use this knowledge to develop strategic interventions that will preserve immune function and disease control. Our on- going analyses in the mouse model show that there is a rapid expansion of early B cell progenitors in the bone marrow and spleen, but the loss of newly formed and mature B cells from these organs, features of dyshematopoiesis that are reversed by clearance of trypanosomes from infected mice by treatment with the trypanocidal drug Berenil. These observations lead us to hypothesize that T. brucei induces a reversible block in B lymphopoiesis that prevents replenishment of the pools of new and mature B cell, which leads to B- cell clonal exhaustion and the collapse of B cell-mediated immunity. We will address this hypothesis in the following specific aims: i) identify the stage at which B lineage development is arrested in Trypanosoma brucei-infected mice ii) determine whether the block in B cell development results from truncated or aberrant B cell differentiation, iii) determine whether blocked B cells resume development when placed in trypanosome- free hosts, or grown in vitro, and if so, whether T. brucei or components of T. brucei prevent escape from the block in development. Methodologies will include: (i) cell surface differentiation antigen profiling using monoclonal antibodies specific for B cell developmental stages, (ii) transcription factor and immunoglobulin gene expression profiling of discrete B cell developmental stages isolated by fluorescence activate cell sorting (FACS) from the spleen and bone marrow of normal and trypanosome-infected mice, (iii) developmental analyses of B cell progenitors FACS isolated as in aim ii, and grown in immunodeficient mice (RAG-/-) and in vitro in the presence and absence of trypanosomes and extracts. Public heath relevance includes the elucidation of the cellular basis for trypanosome-induced B cell exhaustion and the loss of host ability to make protective antibodies responses. Narrative: Loss of the ability to make protective antibody responses and consequently to control parasitemia is a common feature of African trypanosomiasis in people, domestic and laboratory animals. Using the mouse model we have shown an infection-induced reversible block in B lymphocyte development that prevents the maintenance of pools of newly formed and mature B cell leading to B cell clonal exhaustion and thus, collapse of humoral immune competence. We propose to identify the affected B cell developmental stage and the nature of the developmental block as a step towards elucidating the molecular basis of this newly identified mechanism of disease-associated immuno-depression.